Method and apparatus for plating extrusion dies

ABSTRACT

Method and apparatus are set forth for providing a substantially uniform coating on inner wall portions of a die having slots or openings formed in the face of the die. The die is supported by a fixture for up and down movement while it is immersed in a bath of a desired plating solution. A tube or cylinder positioned on an upper surface portion of the die repeatedly collects a column of plating material in a cylinder from the bath on downward movements of the die, so as to provide fresh or replenishment plating solution to the die. The column of plating solution in the cylinder functions as a hydrostatic head and forces the confined solution into the interior of the die on upward movements, thus promoting coating uniformity along inner walls of the die. The die may be rotated 180 degrees so that each face of the die may be subjected to the hydrostatic force flow of the plating solution, also resulting in a more uniform coating.

BACKGROUND OF THE INVENTION

Extrusion dies have been found to be useful in forming cellular or honeycomb ceramic substrates for use in catalytic converters utilized in the exhaust system of internal combustion engines. In order for such converters to function efficiently, it is necessary that the cells provide a substantially large surface area for catalytic material to react with the exhaust gases, and that the cell walls have a substantially thin cross-section dimension so as to provide a substantially large open frontal area and thereby reduce back pressure within the exhaust system.

In order to provide increased surface area within the honeycomb structure so as to enhance catalytic activity, the number of cells has been increased and the wall thickness between the cells has been reduced from about 8 mils to 4 mils or less. Accordingly it became necessary to reduce the slot size of the extrusion die from about 0.008″ to 0.0035″ to form such thin walled cellular structures. To protect the extrusion die from the abrasiveness of the extruding ceramic material, it is necessary to provide a hard protective coating on the walls of the die. The coating must be as uniform as possible for the die to function correctly. The reduced slot size of the dies for thin walled structures, made it difficult to provide a uniform coating on the die walls using the known electroless nickel plating process techniques.

In order for the electroless nickel plating process to produce a uniform coating throughout a die, it is necessary that all surfaces of the die receive a constant replenishment of the plating solution with adequate concentration of the solution constituents. As the plating deposit builds up on the die walls, some of the constituents are consumed and therefore the constant replenishment of the solution to all surfaces of the die is needed to ensure that a uniform deposit thickness is obtained. However, with reduced slot sizes the restrictive nature of the small slots in the dies prevented adequate replenishment of plating solution with known devices, resulting in significant non-uniformity of the coating.

Others have experienced non-uniform plating problems, but for different reasons. In U.S. Pat. No. 4,938,840, coating variations were a problem due to variations in accessibility of the plating solution, whereas in U.S. Pat. No. 6,291,025, the problems in plating thickness were due to the interactions of the plating solution components. U.S. Pat. No. 4,842,886 has a problem with controlling the thickness of the plated layer due to unpredictable surface characteristics, and U.S. Pat. No. 4,406,250 is concerned with the deposition rate variation due to the reaction by-products formed in the plating solution.

The present invention has overcome the problem of plating the walls of extrusion dies for forming thin-walled honeycomb structures with a substantially uniform coating, particularly where the slot size may have a width of less than 8 mils, by implementing an improved fixture and plating solution flow techniques.

SUMMARY OF THE INVENTION

In its simplest form, the present invention is directed to an improved fixture for supporting a die to be plated in a plating solution, and a novel procedure for enhancing the flow of the plating solution through the die to provide a substantially uniform coating on the walls of the die. The fixture is designed for vertical movement within a plating bath and includes a tube or cylinder above the die for replenishing plating solution and for maintaining a column of such plating solution over the die during the plating process, so as to produce uniform deposit thickness, such as from an electroless nickel plating solution. The cylinder complements a peripheral portion of the die and thus the solution within the cylinder, when under a hydrostatic head, is directed solely downwardly through the die and not permitted to flow outwardly around the die. A further feature of the fixture is that the tube may be removed and the die flipped or rotated 180 degrees while within the bath, so that either the pin side or the feed hole side of the die may be facing upward, again to provide better uniformity of the deposited coating.

It thus has been an object of the present invention to provide an improved fixture for facilitating the production of a uniform coating on wall portions of the passageways in dies for forming thin walled honeycomb structures.

A further object of the invention had been to provide an improved method for flowing plating solutions through a die so as to facilitate a uniform coating of the plating material on inner wall portions of the die.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a fixture for holding a die as used in the prior art.

FIG. 2 is a schematic view showing the operation of the prior art fixture of FIG. 1.

FIG. 3 is a micrograph of the coating on a die obtained by utilizing the fixture of FIG. 1, and the operation of FIG. 2.

FIG. 4 is a perspective view of the improved fixture of the present invention.

FIG. 5 is a schematic view of the fixture of FIG. 4 moving downward in a bath of plating solution.

FIG. 6 is a schematic view of the fixture of FIG. 4 moving upwardly in a bath of plating solution.

FIG. 7 is a perspective view of fixture of the present invention, showing the removability of the cylinder portion.

FIG. 8 is a micrograph of the coating on a die produced by utilizing the improved fixture of FIG. 4 and the procedures shown in FIGS. 5 and 6.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

To better appreciate the present invention it is helpful to look at what preceded it. Referring now to FIGS. 1 and 2, a prior art fixture 10 is shown having a lower ring portion 12 for holding a die 14. A pair of side braces or support members 16 are attached at their lower ends to the ring 12, and at their upper ends to a cross bar 18. The cross bar is connected to suitable means (not shown) for moving the fixture 10 and die 14 up and down in a plating bath 20, as shown by arrows a and b. The bath 20 is preferably an electroless nickel plating solution. As the die 14, which is secured in one position by the fixture 10, is moved upwardly and downwardly by the fixture in the bath, a portion of the plating solution 20 flows through the die as shown by arrows c and d, but due to the resistance of the small slot openings in the die, a substantial amount of the flow is over the surface of the die 14 as shown of arrows e and f, rather than through the die as desired. On the up stroke a, flow through the die 14 is shown by arrows c, whereas flow across the die is shown by arrows e, and on the down stroke b, flow through the die is shown by arrows d, whereas flow across the die is shown by arrows f.

FIG. 3 is a micrograph of a section of die 14 showing one side 22 of an extrusion slot after being coated with an electroless nickel plating layer 23 using the prior art method and apparatus shown in FIG. 2. The exit end 24 of the extrusion slot 22 is formed by pin 26 and is shown at the top of the figure, and entrance end 28 of the slot is at the bottom of the figure. The surface of slot 22 may be provided with a recess 29 to enhance flow of the extrusion material through the die 14. As a result of the impedance to flow created by the small openings in the die 14, very little plating occurred on the surface adjacent the bottom portion or entrance end 28 of slot surface 22, whereas a substantial amount is deposited on the surface near the top or exit opening 24 of the slots. Disregarding the recess 29, FIG. 3 shows a coating thickness variation of 51 microns along the extent of die slot 22 between the slot entrance 28 and the exit end 24.

Referring now to FIG. 4, an improved fixture 30 of the present invention is shown including a yoke 32 having trunnions 33 for pivotally mounting a die clamping mechanism 34. The clamping mechanism includes a pair of clamping rings 36 and 38 which encircle a peripheral portion of the die 40 for securing the die therebetween, such as with nuts and bolts 42. Means including yoke 32 and connecting ring 31 move the fixture 30, and accordingly die 40 upwardly and downwardly within a plating bath 56. A lever 44 is secured to the die clamping mechanism 34 and pivotally mounted on the yoke 32 for pivoting or rotating the die 40 180 degrees about trunnions 33, while the die is held by the clamping mechanism 34. The lever 44 has a pair of openings 46 equally spaced from trunnions 33 to receive a locking pin 48 mounted on yoke 32, so as to lock the clamping mechanism 34 in either of the desired 180 degree positions.

A confinement tube or cylinder 50 is removably mounted on the clamping rings 36, 38 by means of pins 52 on the tube 50 which engage t-slots 54 formed in an inner surface of the rings 36 and 38, as shown in FIG. 7. The tube 50 is preferably made of a polymer material, such as cpvc, so as to resist being plated by a plating solution. The tube 50 functions to maintain a confined column of plating solution over the die during the plating process. During the up and down movement of the of the die within the plating solution, the tube fills with fresh plating solution, thus enabling an adequate replenishment of the plating solution in the die so as to produce a uniform thickness deposit on the walls of die slots.

As shown in FIG. 5, when the yoke 32 moves the fixture 30 downwardly within a bath 56 of an electroless nickel plating solution, as shown by arrows v, two things happen, fresh plating solution 56 flows into the tube 50 as shown by arrows w, and plating solution is forced upwardly into the bottom of the die 40 as shown by arrows x. On the upward stroke of the fixture as shown by arrows y, in FIG. 6, all of the plating solution within the tube 50 flows downwardly through the die 40 as shown by arrows z, with none flowing around the die as happened with the prior art fixture. The solution confinement tube 50 produces a column of plating solution with a gravitational weight creating a hydrostatic head of refreshed plating solution over the die 40, which forces a flow of plating solution through the die, while the die remains within the plating bath 56.

By thus enabling adequate solution replenishment throughout the plating process, the uniformity of thickness of the deposit of plating solution along internal passageways of the die between the slot entrance and the exit end of the slots is greatly improved over that obtained with the prior art fixture as shown by die 14 in FIG. 3. FIG. 8 is a micrograph of a portion of a die 40 plated with an electroless nickel plating solution utilizing the improved fixture 30 of the present invention, showing a difference of only 4 microns between the thinnest and thickest portions of coating 23 along the extent of the extrusion slot 58. When compared with the difference in coating thickness of 51 microns obtained on die 14 using the prior art fixture, the improved uniformity of only 4 microns obtained on die 40 with the present invention, represents an improvement over the prior art of 92%.

The confinement tube 50 is removable from the die clamping mechanism 34 by means of the pins 52 on the tube which engage t-shaped slots 54 formed in clamping rings 36, 38, as can be seen in FIG. 7. The tube is removed from the clamping mechanism by simply rotating it so that the two opposing pins 52 on the tube clear the t-slots 54 formed in the clamping rings 36, 38. The ability to remove the tube 50, while the keeping the die 40 submerged in the plating bath 56 is important to the invention, since it provides access to the die 40 for inspection during the plating process, and also enables the die 40 to be flipped over or rotated 180 degrees periodically during the plating process, such that the either the pin side 60 or the feed hole side 62 of the die 40 may be facing upward.

When it is desired to rotate the die 40 from one fixed position to another, the confinement tube 50 is removed from the clamping mechanism 34 by rotating the tube pins 52 to clear the slots 54 in clamping rings 36,38, and the tube is lifted out of the fixture 32. The locking pin 48 is then removed from an opening 46 in pivoting lever 44, and the die held by clamping mechanism 34 is rotated 180 degrees, while the die remains in the plating bath 56, by means of lever 44. The locking pin 48 is then reinserted in an opposite opening 46 in pivot lever 44. The solution confinement tube 50 is then reinstalled on the fixture 30 by positioning the tube pins 52 into their respective t-slots 54 and rotating the tube to lock it within the slots.

Periodically rotating the die 40 in the manner described, further improves the plating thickness uniformity since the direction of flow of the plating solution through the die is alternated. Further, by rotating the die, provides an opportunity for any trapped gas that has built up from the plating process to escape from underneath the die, which aids in solution replenishment throughout the die's internal passageways.

From the foregoing, it can be seen that the improved fixture of the present invention and the procedure for operating the fixture provides for improved uniformity of plating thickness, by not only continually replenishing plating solution to the die, but also by directing and forcing the solution through the die and periodically rotating the die to present an opposite surface of the die to the plating solution.

Although the now preferred embodiments of our invention have been disclosed, it will be apparent to those skilled in the art that various changes and modifications may be made thereto without departing from the spirit and scope thereof as defined in the appended claims. 

1. A method of plating interior wall portions of a die which comprises, immersing a die within a bath of plating solution, moving said die upwardly and downwardly within said bath, repeatedly confining a replenished supply of said plating solution above said die during downward movement of said die within said bath, and flowing said confined supply of replenished plating solution solely into interior portions of said die during upward movement of the die to produce a substantially uniform plating coating on the interior wall portions of said die.
 2. A method as defined in claim 1 including the step confining a column of the replenished plating solution above the die and within the confines of peripheral portions of the die.
 3. A method as defined in claim 1 including the step of creating a hydrostatic head of said plating solution within the confined supply of plating solution and forcing said solution into interior portions of the die.
 4. A method as defined in claim 1 including the step of periodically rotating the die 180 degrees while still being retained within the bath.
 5. A method as defined in claim 1 including the step of periodically flowing the confined supply of replenished plating solution into interior portions of the die from opposite faces of the die.
 6. A method of nickel plating interior wall portions of a die having slots for extruding ceramic honeycomb structures which comprises, immersing an extrusion die to be coated in a bath of electroless nickel plating solution, moving said die upwardly and downwardly within the bath solution, forming a separate confined column of plating solution above the die from the bath, and creating a hydrostatic head within said separate confined column of plating solution; whereby the hydrostatic head forces the confined column of plating solution into the slots of said die to provide a substantially uniform nickel coating on the interior wall portions of said slots.
 7. A method as defined in claim 6 including the step rotating the die 180 degrees while retaining the die within the plating solution.
 8. A method as defined in claim 7 including the step of flowing the confined supply of plating solution under hydrostatic pressure into the die from opposite faces of the die to provide more uniform coating thickness along the slot length.
 9. A fixture for plating interior wall portions of a die member which comprises, means for supporting said die member in a plating bath solution, means for moving said die member upwardly and downwardly within said plating bath and, means for confining a column of said plating solution above said die and for directing the flow of said confined solution solely into interior portions of said die member to provide a substantially uniform coating of plating material along said interior wall portions.
 10. A fixture for plating interior wall portions of a die member as defined in claim 9, wherein said confining means includes means for replenishing plating solution to an upper surface of said die member.
 11. A fixture for plating interior wall portions of a die member as defined in claim 9, including pivotal means for rotating said die member from one fixed position to another fixed position while said die is in said plating bath.
 12. A fixture for plating interior wall portions of a die member as defined in claim 9, wherein said means for supporting said die includes a pair of clamping rings which encircle a peripheral portion of said die member.
 13. A fixture for plating interior wall portions of a die member as defined in claim 12, wherein said confining means includes a tubular member removably secured to said support means.
 14. A fixture for plating interior wall portions of a die member as defined in claim 9 wherein said confining means includes a cylindrical member removably secured to at least one of said clamping rings.
 15. A fixture for plating interior wall portions of a die member as defined in claim 9 including trunnion means pivotally mounting said die support means and wherein said pivotal means includes a lever secured to said die supporting means for rotating said support means and said die about said trunnion means.
 16. A fixture for plating interior wall portions of a die member as defined in claim 9 including locking pin means for locking said die in a desired rotated position.
 17. A fixture for plating interior wall portions of a die member as defined in claim 13 wherein said clamping rings have recessed t-slots and said cylindrical member has a pair of pins for engagement with said slots for securing said cylindrical member on said clamping ring. 